Electrophotographic printing is well known and has been widely refined. For example, today, almost every office and indeed some homes have electrophotographic copiers. The industry has grown to the point where it is now a highly competitive multibillion dollar industry. In most instances, these home and office copies are capable of providing only about a few copies per minute.
In electrophotography, images are photoelectrically formed on a photoconductive layer mounted on a conductive base. Liquid or dry developer or toner mixtures may be used to develop the requisite image.
Liquid toner dispersions for use in the process are formed by dispersing dyes or pigments and natural or synthetic resin materials in a highly insulating, low dielectric constant carrier liquid. Charge control agents are added to the liquid toner dispersions to aid in charging the pigment and dye particles to the requisite polarity for proper image formation on the desired substrate.
The photoconductive layer is sensitized by electrical charging whereby electrical charges are uniformly distributed over the surface. The photoconductive layer is then exposed by projecting or alternatively by writing an image over the surface with a laser, L.E.D., or the like. The electrical charges on the photoconductive layer are conducted away from the areas exposed to light with an electrostatic charge remaining in the image area. The charged pigment and/or dye particles from the liquid toner dispersion contact and adhere to the image areas of the plate. The image is then transferred to the desired substrate such as a carrier sheet.
In contrast to office and home copiers, high speed electrophotographic printing presses are being developed wherein successive images are rapidly formed on the photoconductive medium for rapid transfer to carrier sheets or the like traveling at speeds of greater than 100 ft./min. and even at speeds of from 300-500 ft./min. As can be readily understood, such high speed machines rapidly consume the solid pigment and/or dye and associated resin particles from the liquid toner baths in order to develop the multitude of latent electrostatic images needed for successful operation of the printing press. Unlike the situation in office and home copies in which an operator can simply replenish consumed liquid toner dispersion by shutting the copier off, opening its housing and replacing the spent liquid toner dispersion with a container of fresh toner dispersion, it is clearly unacceptable to shut down a high speed printing process to make such replacement in light of the rapid consumption of the liquid toner material and hence the frequent replacement thereof. An automated liquid toner supply system is accordingly desirable.